This invention relates in general to leveling or flattening sheet metal and more particularly to a machine and process for leveling sheet metal that is supplied in a continuous strip, with such leveling occurring by stretching the strip over its entire width and without severing it into segments.
Many manufactured goods, such as automobiles, appliances, and office furniture, to name a few, contain substantial quantities of sheet steel which is purchased directly from steel mills, or perhaps from intermediate processors that unwind coils of mill steel, slit it and then rewind it into coils of lesser width. Upon being coiled, the sheet steel takes on a set or curvature which should be removed to convert the sheet steel into useful products, since practically all stamping and blanking operations require flat sheet as the starting material. So-called straighteners are available for this purpose. The typical straightener has a series of rolls into which the steel strip containing the coil set is fed. The rolls impart an undulated configuration to the sheet steel as it passes between them, but upon emerging from the rolls the steel is substantially flat.
Steel sheet, however, acquires additional distortions at the mill as well as during slitting. For example, often one or both edges of the sheet are longer than the center, producing an undulated edge often referred to as edge wave. On the other hand, the center may be longer than the edges, and this creates a bulge in the sheet a condition sometimes called the oil can effect. Machines do exist for eliminating the edge wave and the oil can effect.
One such machine resembles the simple straightener in that it possesses a series of rolls between which the sheet passes in a somewhat undulated manner. However along each roll are back-up bearings which are capable of deflecting the roll against which they bear. Thus, if the steel sheet passing between the rolls contains the edge effect, the rolls are deflected at their centers to stretch the center of the sheet while leaving the edges at their original length. The stretching causes the steel sheet along its center to exceed the elastic limit of the steel, so that the steel yields in this region with the amount of yield being enough to equalize the sheet dimensionally between its center and edges. Similarly, if the sheet possesses an oil can effect, the rolls are deflected more at their edges. The end result is a reasonably flat sheet of steel. This type of leveling machine, while being moderate in cost, is extremely difficult to operate and indeed calls for the exercise of a considerable amount of judgment. In short, the machine requires a highly skilled operator.
Another type of machine known as a stretcher-leveler is somewhat simpler than the flexible roll machine and less complex to operate, in that it involves simply stretching the sheet beyond its elastic limit across the entire width of the sheet. However, the segment which is to be stretched must first be severed from the distorted strip and then gripped at its ends in the jaws of C-clamps. The C-clamps are then spread apart to stretch the severed segment. The clamps of course must grip the segment tightly, and this leaves marks along the end edges of the sheet. These marks will show through painted finishes and as a consequence the edges of such stretched segments are often trimmed. Moreover, the stretching changes the length of the severed segment, making trimming all the more necessary. Thus, stretching steel or other metal sheet in the typical stretcher-leveler is a time consuming and labor intensive process
Continuous tension leveling machines, which are also currently available, stretch the sheet metal strip across the entire width of the strip without having to sever the strip into segments, and these machines have gained acceptance at mills and other processors which sell coiled steel. In these machines the strip after paying off of the coil passes between two sets of rolls with the rolls of each set being arranged as capstans so as to actually grip and drive the continuous strip. Moreover, the rolls of the downstream set rotate at a slightly greater velocity than the rolls of the upstream set, so that the steel is stretched in the region between the two sets of rolls - indeed stretched beyond its elastic limit. By reason of the stretching the strip emerges from the second set of rolls in a leveled condition. These machines are extremely costly. Also, since they operate continuously, they cannot be utilized in conjunction with intermittently operated devices such as shears for severing continuous strip into useful lengths of steel sheet.
A need exists for a leveler which operates on a continuous strip of sheet steel, aluminum or other metal without severing the strip into segments and which is further available at a moderate cost.